DNA Structure and Function

Nucleic acids are polymers of nucleotides (NEW-clee-oh-tides). A nucleotide consists of a sugar, a phosphate group, and a single- or double-ringed nitrogenous (ny-TRODJ-eh-nus) base. Three bases—cytosine (C), thymine (T), and uracil (U)—have a single carbon-nitrogen ring and are classified as pyrimidines (py-RIM-ih-deens). The other two bases—adenine (A) and guanine (G)—have double rings and are classified as purines (fig. 4.2). The bases of DNA are C, T, A, and G, whereas the bases of RNA are C, U, A, and G.

The structure of DNA resembles a ladder (fig. 4.3a). Each sidepiece is a backbone composed of phosphate groups alternating with the sugar deoxyribose. The steplike connections between the backbones are pairs of nitrogenous bases. Imagine this as a soft rubber ladder that you can twist, so that the two backbones become entwined to resemble a spiral staircase. This is analogous to the shape of the DNA molecule, described as a double helix.

The nitrogenous bases face the inside of the helix and hold the two backbones together with hydrogen bonds. Across from a purine on one backbone, there is a pyrimidine

Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

Germ Layers Human Embryo Mcgraw Hill

Chromatin i'V^E

Supercoiled structure (200 nm in diameter)

Chromatin i'V^E

diameter)

DNA (2 nm in diameter)

Figure 4.1 Chromatin Structure. (a) Nuclear contents of a germ cell from an 8-week-old human embryo (colorized SEM). The center mass is the nucleolus. It is surrounded by granular fibers of chromatin. Each granule is a nucleosome. (b) The coiling of chromatin and its relationship to the histones. Supercoiling beyond the 10-nm level occurs only during mitosis.

DNA (2 nm in diameter)

Adenine

Adenine

H

H

OH

Deoxyribose

Phosphate

Deoxyribose

Purines

HN C NH

/ nh2

Adenine (A)

Guanine (G)

Pyrimidines

HC NH

NC NH C

O

Cytosine (C)

O

Thymine (T)

HN

II

OC O

N H

Uracil (U)

Figure 4.1 Chromatin Structure. (a) Nuclear contents of a germ cell from an 8-week-old human embryo (colorized SEM). The center mass is the nucleolus. It is surrounded by granular fibers of chromatin. Each granule is a nucleosome. (b) The coiling of chromatin and its relationship to the histones. Supercoiling beyond the 10-nm level occurs only during mitosis.

Figure 4.2 Nucleotides and Nitrogenous Bases. (a) The structure of a nucleotide, one of the monomers of DNA and RNA. In RNA, the sugar is ribose. (b) The five nitrogenous bases found in DNA and RNA nucleotides.

Saladin: Anatomy & I 4. Genetics and Cellular I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

132 Part One Organization of the Body

Figure 4.3 DNA Structure. (a) The "twisted ladder" structure. The two sugar-phosphate backbones twine around each other while complementary bases (colored bars) face each other on the inside of the double helix. (b) A small segment of DNA showing the composition of the backbone and complementary pairing of the nitrogenous bases. (c) A molecular space-filling model of DNA giving some impression of its actual geometry. How would the uniform 2-nm diameter of DNA be affected if two purines or two pyrimidines could pair with each other?

Figure 4.3 DNA Structure. (a) The "twisted ladder" structure. The two sugar-phosphate backbones twine around each other while complementary bases (colored bars) face each other on the inside of the double helix. (b) A small segment of DNA showing the composition of the backbone and complementary pairing of the nitrogenous bases. (c) A molecular space-filling model of DNA giving some impression of its actual geometry. How would the uniform 2-nm diameter of DNA be affected if two purines or two pyrimidines could pair with each other?

on the other. A given purine cannot arbitrarily bind to just any pyrimidine. Adenine and thymine form two hydrogen bonds with each other, and guanine and cytosine form three, as shown in figure 4.3b. Therefore, wherever there is an A on one backbone, there is a T across from it, and every C is paired with a G. A—T and C-G are called the base pairs. The fact that one strand governs the base sequence of the other is called the law of complementary base pairing. It enables us to predict the base sequence of one strand if we know the sequence of the complementary strand. The pairing of each small, single-ringed pyrimidine with a large, double-ringed purine gives the DNA molecule its uniform 2-nm width.

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